Once you have speakers, you may want a separate headphone amp - if so, your cans are ideally suited for the Crack.
We only have one speaker kit, and it's kind of overkill for the little SEX amp. But there are many speaker kits from places like Madisound or Parts Express. Or the SEXy Speaker DIY project, filed under Product Archives/Community.
You already have the only S.E.X. kit upgrade. There's the power cord kit, of course.
If you only have one source then you don't need a preamp.